The present invention relates to apparatus used to drive a multi-phase motor from a direct current source. In particular it considers not the detailed circuitry, but rather the method and arrangement of the power components and electrical switches used.
In the design of inverters for operating multi-phase motors from a DC power source, various types of solid-state or semiconductor switches are employed. Although modern semiconductor switches, such as MOS-FETs have very low losses, the problems caused by the heat losses of the switching devices cannot be avoided when motors of even modest power are controlled.
Traditionally this problem has been solved by employing large heat sinks. These require space that is not always available. Alternatively, liquid cooling has been successful in some cases, but the physical problems of large power conductors used to connect to the high capacity switches remain.
It is a major object of this invention to spread out the sources of heat so as to minimize local heating. In this way the invention allows commercially available components to be combined to produce higher overall power conversion and at higher efficiencies, as well as to minimize the overall size of the conversion apparatus by dividing the requirements among several smaller devices rather than a single massive one.
Objects therefore are to allow the use of multiple smaller switches and electrical conductors in inverters, to allow the use of printed wiring boards in power areas where they would not often be considered feasible, and to remove the heat loss at a multiplicity of points with low temperature rise at each, rather than at a few points with a small number of massive heat sinks.
A further object is to permit certain monitoring of power flow so that the currents and sources of heat can be distributed optimally, and so to compensate for the variations normal in commercially available components.
xe2x80x9cBrushless DC Motorsxe2x80x9d have been available for several years, along with the circuitry for driving them. These systems employ various types of semiconductor switches and switch combinations to convert a direct current source of power into the multi-phase alternating current required by the motor(s). The control switching semiconductors are usually selected to match the voltage and current requirements of the motor, and are mounted to a heatsink which is appropriately cooled so as not to exceed the thermal ratings of the switches.
Usually the power semiconductor heatsink is air cooled, using a fan if necessary to remove the heat. The power supply is connected to the switches by conductors of sufficient size that they can carry the requisite current without themselves contributing excess heat.
In practice, if sufficiently capacious switches are not available several smaller ones may be combined to achieve the higher rating required. However, these arrangements have in the past required greater physical space as well as larger electrical conductors for their interconnection. Also, as the heat sinks become larger physically the electrical inductance is increased and other switching problems develop.
Past systems have relied on xe2x80x9cload splittingxe2x80x9d or sharing of the capabilities of the various switching elements either on the basis of their being matched physically, being selected for matching characteristics, or by the use of dissipative load-sharing resistors.
An objective of the present invention is to allow the use of multiple switches in a power inverter, so that the sources of heat can be separated from each other. This allows the use of smaller and less expensive devices, as well as physically realizable devices, in the fabrication of high power inverters.
Another objective is to improve the cooling of the switching elements by permitting multiple heat flow paths from the various switching elements to the heat sink(s).
Another objective is to allow the use of multiple reasonably thin electrical conductors in parallel, rather than the more unwieldy heavy equivalent conductors generally employed.
Another objective is to reduce the inductance of the switching arrangement (and thereby increase its effective switching speed) by allowing each of the switching circuits to be essentially self-contained.
Another objective is to allow monitoring of the power being controlled by each switch, so that the switching signals can be modified to allocate power among them in the most appropriate manner.
Another objective is to actually modify the power switching as described above either to distribute the heat to all switching units equally, or to limit the maximum power in the totality of switches to a safe or appropriate level, since it is well known that most components will experience a longer life if operated below their maximum ratings.
Accordingly, a preferred embodiment consists of a plurality of switches and switching elements, each of which is provided with its own power supply, load conductors, heatsink and current sensing resistor; a logic system for controlling the turn-on and turn-off of the switches as appropriate, based on both command information and feedback from the current sensing resistors, voltage across the closed switch and temperature sensors at each heatsink; and a heatsink suitably arranged to carry off the heat and to provide a safe and reasonable temperature environment for operation of the switches.
Thus, the present invention presents an improvement over existing arrangements, wherein a plurality of electronic switches are arranged and/or mounted in a manner so as to distribute the switch heat losses among a plurality of locations. Each of the electronic switches is provided with its own connections to the power supply, as well as to the motor being controlled. The use of equal length conductors from the power supply to the switches and to the load tends to enhance the equality of load sharing among the multiple electronic switches. This also enables the use of thinner and more flexible conductors, which are more easily handled and terminated, and which permits them to be terminated to a printed wiring board. Printed board mounting is especially important, since the ancillary circuitry needed for control and protection of the power devices can be mounted to the board as wellxe2x80x94in close proximity, and with minimal wiring between them. In particular, each switch can have its own power supply filter capacitor (from which surges of current may be drawn) close by. In addition, a smaller capacitor may be mounted directly across the electronic switch (and on the printed board) which can constrain switching transients to the area of the switch itself, preventing them from radiating and in particular preventing them from reaching as far as the current sensing resistor which is used for monitoring the switch current. The novel arrangement allows the electronic switches to be mounted around a portion of the periphery of the motor frame so as to use the motor frame as a heatsink. The arrangement around the motor frame periphery is not critical. A symmetrical arrangement can be used, and such an arrangement provides for good distribution of heat. However, an asymmetric arrangement can also be employed. This use of the motor frame as a heatsink is particularly advantageous if the motor frame is submerged in a fluid, or if it can be liquid cooled. Additional electronic switches can be accommodated by xe2x80x9cdoubling upxe2x80x9d the mounting in pairs, which requires the use of an intermediate heat distributing body to conduct the heat from the electronic switch elements to the motor frame. These can be used as additional switches, or the pairs of elements can be connected electrically in parallel to form each switching element. The arrangement also allows means for sampling or measuring the actual voltage drops across the electronic switches when in their closed state. This voltage drop can be used by the controller to compensate for variations in the xe2x80x9conxe2x80x9d resistance of the various electronic switches, and to alter the duty cycles so as to equalize the heat load among them or to shut the system down if this can not be accomplished. To aid in control, the switch output currents can be monitored by an ohmic sensing resistor of low wattage, which can also be mounted to the printed board.